Rubber-modified terpolymer with improved molding characteristics

ABSTRACT

Rubber-modified terpolymers of from 50 to 83% of a monoalkenyl aromatic monomer, from 15 to 30% of an unsaturated dicarboxylic acid anhydride, from 2 to 20% of an unsaturated nitrile are described which are useful as molding compositions.

This application is a continuation-in-part of application Ser. No.865,049, filed Dec. 27, 1977.

BACKGROUND OF THE INVENTION

This invention relates to molding compositions and particularly tomolding compositions that cure during thermal treatment and which cannevertheless be thermoformed.

For most molding compositions based on thermoplastic polymers, theshaping operation does not involve any radical change in molecularstructure, that is to say, it remains thermoplastic. The consequence ofthis thermoplastic character is that the mechanical properties such asmodulus will decrease with increasing temperature and the polymer willusually have a low resistance to organic solvent such as might be usedas paint vehicles for example.

If a thermosetting resin is chosen, the solvent and modulus deficienciesare considerably reduced but the resin cannot be thermoformed byconventional thermoplastic fabrication techniques. Furthermore, highmodulus thermosetting resins all suffer from a lack of toughness whichprecludes their use in many applications. By contrast the polymers ofthe invention not only have good solvent resistance and high modulus buthave in addition an unexpectedly high toughness.

The polymer compositions of the invention therefore, provide many of theadvantages of both thermoplastic and thermosetting resins in that thepolymers are tough and a broad range of thermoplastic forming operationscan be used and the thermoformed polymer can be given good solventresistance and retention of modulus by appropriate selection of reactionconditions.

The polymer is believed to acquire these properties through crosslinkingand the amount that occurs can be controlled by variation of thecomposition of the polymer and the heat history of the polymer duringprocessing.

The polymer of the invention thus provides a hybrid between truethermoplastic and thermosetting polymers that has many of the bettercharacteristics of both.

DISCUSSION OF THE PRIOR ART

The copolymerization of styrene and acrylonitrile in the presence of arubber is very well known in the art and indeed provides one processvariant available for the production of the well known ABS polymer.

It is also known that styrene and maleic anhydride can be copolymerizedin the presence of rubber and a process for producing such polymer isdescribed in U.S. Pat. No. 3,919,354.

It is further known that the polymers described in U.S. Pat. No.3,919,354 can in addition comprise a copolymerizable termonomer such asacrylonitrile in amounts from 10 to 40% and preferably from 20 to 35% ofthe polymer composition.

It is also known from U.S. Pat. No. 2,439,227 that styrene, maleicanhydride and acrylonitrile can be copolymerized in certain proportionsto produce a high molecular weight polymer that possesses a good degreeof tensile and flexural strength by very low impact strength. Suchpolymers cannot however, be readily formed as they rapidly becomeinfusible at temperatures in excess of 120° C. and are thus inaccessibleto normal shaping techniques and even polymer isolation operationsinvolving heat treatment.

A polymer composition has now been found that can withstand thesubstantial heat treatments involved in the conventional drying andcompounding of a thermoplastic polymer without becoming so crosslinkedas to make it impossible to thermoform.

The compositions of the invention have remarkable resistance to solventsafter being heat treated and retain their shear modulus even at elevatedtemperatures in a most unexpected fashion. In addition to the aboveadvantages it is found that the polymers have excellent impact strengthwhich greatly exceeds that which might have been expected on the basisof experience with analogous polymers.

Finally, the polymers have improved resistance to burning since onexposure to a flame they form a surface char which tends to inhibit theaccess of air to the combustion area.

DESCRIPTION OF THE INVENTION

The present invention provides a polymeric molding composition obtainedby polymerizing a monomer mixture comprising from 50 to 83% of avinylaromatic monomer, from 15 to 30% of an unsaturated dicarboxylicacid anhydride and from 2 to 20% of an unsaturated nitrile, allpercentages being by weight, in the presence of from 2 to 30 parts byweight, based on the weight of the polymer composition, of a rubberhaving a glass transition temperature below 0° C. which compositionremains thermoformable after having been exposed to temperatures of 190°C. for 30 minutes and thereafter Brabender compounded for 2 minutes.

The invention further comprises a process for the production of apolymeric molding composition which comprises polymerizing from 50 to83% by weight of a vinylaromatic monomer, from 15 to 30% by weight of anunsaturated dicarboxylic acid anhydride and from 2 to 20% by weight ofan unsaturated nitrile in the presence of from 2 to 30% by weight of arubber having a glass transition temperature below 0° C. to form arubber substrate grafted with a superstrate terpolymer and dispersed ina matrix terpolymer; said process comprising forming a solutioncomprising a vinylaromatic monomer, the unsaturated nitrile monomer andthe rubber; catalytically initiating polymerization of the mixture at atemperature below 120° C. with continuous addition of the unsaturateddicarboxylic acid anhydride during the polymerization reaction so as toproduce a rubber modified terpolymer of uniform composition that remainsthermoformable after having been exposed to temperatures of 190° C. for30 minutes and thereafter Brabender compounded at 240° C. for 2 minutes.

It is understood that the majority of the polymers within the purview ofthis invention are capable of tolerating a heat history comprisingheating at temperatures in excess of 190° C. for longer than 30 minutesfollowed by Barbender compounding at 240° C. for more than 2 minutes.These conditions are given as a threshhold heat history that a polymermust be able to tolerate and still remain thermoformable in order to beclassed as polymeric compositions according to the present invention.

The compositions of the invention are therefore characterized by theirability to withstand considerable heat treatment without becomingcrosslinked to the extent that they can no longer be thermoformed usingstandard commercial techniques. The un-modified polymers of U.S. Pat.No. 2,439,227 whose compositions are specified only in terms of monomerscharged begin to crosslink at 120° C. and become infusible. It has beendiscovered that if the final polymer composition is maintained withincertain limits and specifically if the unsaturated nitrile content ismaintained below about 20%, the reaction conditions can be controlledsuch that a moldable composition is obtained that can be left at 120° C.for several days without rendering it infusible and which can in fact beexposed to drying conditions at 190° C. or more for at least 30 minutesfollowed by a Brabender compounding at 240° C. for a further 2 minutesand still be thermoformable under standard thermoplastic polymer moldingconditions. This is indeed surprising and the advantageous resultsoutlined above are shown by only a narrow range of compositions withinthe broad disclosure of the prior art.

What is more, it is important that the conditions of the reaction toproduce the polymeric molding composition of the invention be carefullytailored towards their production by the use of a late additiontechnique for the anhydride component. It is found that straight masspolymerization techniques do not result in a uniform polymer because ofthe different reactivities of the monomers involved unless thecomposition falls within a very narrow range of azeotropic compositions.By the late addition technique the unsaturated dicarboxylic acidanhydride, which has a strong tendency to polymerize in a 1:1 ratio withthe vinylaromatic monomer, is kept in low concentration and arubber-modified terpolymer of uniform composition is formed.

The polymerization process should preferably be conducted attemperatures below about 120° C. and in practice this means thatstraight thermal polymerizations are disfavored. This is because theyrequire higher temperatures or an undesirable combination of moderatetemperatures maintained over a prolonged period.

The preferred processes of the invention therefore employ a catalyticinitiator such a azobisisobutyronitrile or a perester such as tert-butylperoctoate, tert-butyl peracetate or tert-butyl perbenzoate alone or inadmixture. The reaction temperature is preferably maintained below about120° C. and most preferably in the range of 85°-100° C. The totalreaction time is generally from 5 to 10 hours such as from 5 to 8 hours.

The polymer is commonly produced in solution on account of thedifficulty of introducing the anhydride component, though in somecircumstances a viable mass polymerization process which does notrequire a solvent might be feasible.

The separation of the polymer from the solvent can be by any sultablemeans including precipitation and removal of volatile material byheating under reduced pressure.

During the polymerization of the monomers in the presence of the rubber,a minor proportion reacts to form a superstrate polymer grafted onto thesubstrate rubber. However, the major proportion of the monomerspolymerize together to form a matrix polymer without becoming grafted tothe substrate and it is understood that the polymeric composition of theinvention comprises matrix polymer as well as grafted substrate rubber.

The polymerization method described above provides a technique forobtaining rubber-modified terpolymers but it does not guarantee that thepolymers obtained will have the outstanding characteristics of thepolymers of the invention. As the proportion of the unsaturated nitrilemonomer in the composition rises, so too does the tendency of thecomposition to crosslink. This tendency can however, be controlled, upto a nitrile content of about 20% by precisely controlling thecomposition of the polymerizing mixture using the continuous monomerfeed technique described above and by the use of chain transfer agentsto maintain the matrix molecular weight at a suitable level. Generallyit is desirable that the intrinsic viscosity of the matrix polymer bymaintained within a range of 0.20 to about 0.65 and preferably from 0.35to 0.65 deciliters/gram, measured at 25° C. in methyl ethyl ketonesolution.

Suitable chain transfer agents include mercaptans such as tertiarydodecyl mercaptan; long chain unsaturated hydrocarbons such asterpinolene, myrcene, d-limonene or other terpene; norbornene, indeneand other generally known chain transfer agents. The amount used isgenerally less than 1% by weight of the composition.

The rubber substrate component of the polymeric composition can beselected from a wide range of alternatives including butadiene polymersand copolymers, polyisoprene, polychloroprene, polyacrylate rubbers, andethylene/propylene/diene rubber (EPDM), polypentenamer andethylene/vinyl acetate rubbers. Copolymers of cyclopentene with a minorproportion of a non-cyclic α-olefin such as for example a copolymer of55 to 90% of cyclopentene with from 5 to 45% of ethylene areparticularly useful. Other rubbers which have a Tg below 0° C. and whichmay be grafted with the monomers used to produce the polymericcomposition can readily be supplied by the skilled reader. The preferredrubbers have a Tg below about -30° C. and the most preferred arepolybutadiene, especially the high-cis butadienes, and copolymers ofbutadiene with up to 40% by weight of a styrene or acrylonitrilecomonomer.

The monoalkenyl aromatic monomer is preferably styrene but styrenederivatives such as chlorostyrene, vinyl toluene, alpha-methyl styrene,alpha-methyl vinyl toluene, 2,4-dichlorostyrene and2-chloro-4-methylstyrene may be substituted for styrene in whole or inpart if desired.

The unsaturated dicarboxylic acid anhydride is most preferably maleicanhydride though any of the homologues of maleic anhydride such asitaconic, citraconic, and aconitic anhydrides can also be used.

The preferred polymeric molding composition of the invention comprises amatrix polymer and dispersed therein a rubber substrate polymer having aglass transition temperature (Tg) below -30° C. with a superstratepolymer grafted thereon, wherein the matrix and superstrate polymerstaken together comprise from 56-78% by weight of a monoalkenyl aromaticmonomer; from 20-30% by weight of an unsaturated dicarboxylic acidanhydride and from 2 to 14% by weight of an unsaturated nitrile.

The preferred unsaturated nitrile monomers are acrylonitrile andmethacrylonitrile. The preferred amount of the nitrile monomer in thesuperstrate/matrix polymer part of the composition is from 2 to 14 partsby weight since this gives a greater flexibility of pre-thermoformingprocessing, very good impact strength and distortion temperatures andsufficient crosslinking after thermoforming to give excellent shearmodulus retention at high temperature.

The polymeric composition is conveniently prepared by dissolving therubber in a solution of the monovinyl aromatic component and theunsaturated nitrile monomer in a suitable solvent, and then polymerizingthe solution with late addition of the anhydride component in the mannerdescribed in, for example, U.S. Pat. Nos. 2,971,939, 3,336,267 and3,919,354 which are incorporated herein by reference.

A polymerization schedule for the late addition of the anhydride may bedevised on the basis of the relative reactivities of the monomers.Typical schedules involve preparing an initial reaction mixturecomprising a solvent, the bulk of the alkenyl aromatic monomers, a verysmall amount (or none) of the anhydride monomer and the major proportionof the nitrile. The rubber is dissolved in this mixture and the balanceof the monomers is added slowly during the polymerization.

The amount of rubber/substrate (ungrafted basis) in the polymericcomposition, which as indicated above, includes both the graftedsubstrate and the matrix polymer, is in the range of from 2 to 30% byweight based on the weight of the polymeric composition. Preferably,however, the rubber substrate represents from 5 to 25% of the weight ofthe polymeric composition.

The composition can contain other additives such as for example,additional ungrafted rubber components, flame retardants, smokesuppressants, antioxidants, stabilizers, lubricants, antistaticadditives, colorants and fillers such as glass fibers or mineralparticles.

A wide range of antioxidant stabilizers for styrenic polymers isavailable but particularly satisfactory results can be obtained using1,3,5-trimethyl-2,4,6-tris[3,5-di-tert-butyl-4-hydroxybenzyl]benzene and2,2,'-methylene-bis-(4-methyl-6-tert-butyl phenol) terephthalate. Therubber component usually requires a different stabilizer such as atris(alkylphenyl)phosphite though the prior art can supply others thatwould be effective.

The total amount of stabilizer that may be used is not critical buttypically up to 5% by weight based on the total composition is found tobe adequate. In general 0.1 to 2% is the range chosen for practicaladvantage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is now more particularly described with reference to thefollowing examples which are for the purpose of illustration only andare intended to imply no limitation thereon.

EXAMPLE 1

This Example describes the process used to prepare a rubber modifiedpolymer of styrene, maleic anhydride and acrylonitrile.

An agitated resin kettle was charged with 710 g of styrene, 54 g ofacrylonitrile, 2.3 g terpinolene (a chain transfer agent) and a solventmixture consisting of 300 g of methyl ethyl ketone (MEK) and 300 g oftoluene. In this mixture were dissolved 90 g of polybutadiene rubber and0.5 g trisnonylphenyl phosphite (a stabilizer). A solution of 23 g ofmaleic anhydride in 100 ml of MEK and a mixture of 0.2 cc of t-butylperacetate and 0.3 cc of t-butyl peroctoate were then added to the abovemixture. The temperature of the reaction mixture was raised to 85° C.and maintained at this level and continuous late addition of a solutionof 144 g of maleic anhydride and 26 g acrylonitrile in 200 g MEK and 100g toluene was initiated. The continuous late addition took place over a61/2 hour period.

A slug addition of 0.2 cc of t-butyl peracetate was made at 2 and againat 4 hours from the start of the continuous late addition. A sample ofthe polymer (designated A) was then separated from the solvent byplacing the solution in an oven at 220°-230° C. for 1.5 hours undervacuum.

A further sample (designated B) of the same polymer was separated fromunreacted monomer and solvent by precipitation from n-hexane at roomtemperature.

The ratio of the monomers styrene/maleic anhydride/acrylonitrile in thepolymeric compositions was found to be 68/26/6 and the proportion ofrubber was 14.3%. The dried polymers were then each compounded in aBrabender with 0.2 parts by weight of1,3,5-trimethyl-2,4,6-tris(3,5-ditertiarybutyl-4-hydroxybenzyl)benzene(an antioxidant for the polymer) for 3 minutes at 240° C., after whichthey were compression molded at about 240°-250° C.

The shear modulus of each sample was then determined at temperatures of25° C. and 120° C. The results obtained appear in Table 1 below. Theshear modulus was determined using a mechanical spectrometermanufactured by Rheometrics, Inc. employing the standard procedurerecommended by the manufacturer.

                  TABLE 1                                                         ______________________________________                                        TEMPERATURE DEPENDANCE OF SHEAR MODULUS                                                         Shear                                                                         Modulus (dynes/cm.sup.2)                                                      25° C.                                                                         120° C.                                      ______________________________________                                        RM-SMA    (76/24)        8.9 × 10.sup.9                                                                   4.7 × 10.sup.9                        RM S/MA/AN                                                                              (68/26/6) Sample A                                                                          10.1 × 10.sup.9                                                                   7.0 × 10.sup.9                        RM S/MA/AN                                                                              (68/26/6) Sample B                                                                          10.1 × 10.sup.9                                                                   7.2 × 10.sup.9                        ______________________________________                                    

The comparison of the polymeric composition of the invention with onecontaining no acrylonitrile is particularly instructive. Not only is theshear modulus substantially higher at both temperatures, but theretention of shear modulus at the elevated temperatures is very muchbetter. This is somewhat surprising and is believed to result fromcrosslinking of the polymer chains during molding.

There is a further significant lesson from Table 1 in that the polymerthat had been subjected to devolatilization in an oven under vacuum at230° C. for 1.5 hours did not appear to be noticeably different from thesample (B), which had a much less severe heat history.

In the Examples that follow the basic preparative method used was thatdescribed in Example 1. The appropriate monomer addition schedule isderived from a computer model based on the monomer reactivities. Typicalschedules are given in Table 2.

                  TABLE 2                                                         ______________________________________                                        MONOMER ADDITION SCHEDULE RATIOS                                                                            LATE                                            TARGET POLYMER INITIAL CHARGE ADDITION                                        S/MA/AN        S/MA/AN        S/MA/AN                                         ______________________________________                                        60/30/10 (Ex. 1)                                                                             60/4.7/18.7    0/25.3/1.3                                      63/25/12       63/3.3/10      0/21.7/2.0                                      65/20/15       65/2.6/13.1    0/17.4/1.9                                      67/17/16       67/2.0/13.6    0/15.0/2.4                                      ______________________________________                                    

In each case the amounts given are percentages based on the finalpolymer weight.

EXAMPLES 2-13

These Examples record the properties of polymers prepared by processeswhich were essentially the same as the one described in Example 1. Againfor each composition one sample was prepared using oven devolatilizationand another by precipitation from n-hexane. In both cases the sametechniques were used as are set forth in Example 1 and the oven- andhexane-isolated samples are described as A and B, respectively.

The preparative conditions for each are set forth in Table 3 and thesolubility properties of the polymers obtained appear in Table 4.

                  TABLE 3                                                         ______________________________________                                        PREPARATION OF RUBBER MODIFIED TERPOLYMERS                                           Composition                   %                                        Example                                                                              S/MA/AN     Chain Transfer Agent (phr)                                                                      Rubber                                   ______________________________________                                        *2     68/26/6     0.1     Terpinolene 14.3                                   3      66/28/6     0       --          15.8                                   4      68/26/6     0.23    Terpinolene 15.4                                   5      65/24/11    0.23    Terpinolene 16.0                                   6      69/20/11    0.23    Tertiary dodecyl                                                                          14.0                                                              mercaptan                                          7      66/23/11    0.46    Terpinolene 13.4                                   8      63/23/14    0       --          14.3                                   9      66/20/14    0.23    Tertiary dodecyl                                                                          12.2                                                              mercaptan                                          10     56/25/19    0       --          15.6                                   11     55/26/19    0.23    Tertiary dodecyl                                                                          13.5                                                              mercaptan                                          +12    56/25/19    0       --          15.6                                   13     54/23/23    0.23    Tertiary dodecyl                                                                          13.7                                                              mercaptan                                          ______________________________________                                         *Repeat of Example 1.                                                         +Brabender compounded for 10 minutes instead of three minutes.           

                  TABLE 4                                                         ______________________________________                                        SOLUBILITY PROPERTIES OF POLYMERIC                                            COMPOSITIONS OF TABLE 3                                                                %    Solution      Solubility(2)                                     Example                                                                              Sample  AN     Viscosity dl/g(1)                                                                         Before After                                ______________________________________                                        2      A       6      0.55        D      D                                           B       6      0.52        D      D                                    3      A       6      (Not measured)                                                                            D      D                                           B       6      0.6         D      D                                    4      A       6      0.48        D      D                                           B       6      0.46        D      D                                    5      A       11     0.65        D      PD                                          B       11     0.60        D      D                                    6      A       11     0.50        D      PD                                          B       11     0.48        D      D                                    7      A       11     0.58        D      D                                           B       11     0.48        D      D                                    8      A       14     Insol.      ND     ND                                          B       14     Insol.      PD     ND                                   9      A       14     --          PD     ND                                          B       14     --          PD     SS                                   10     A       19     --          SS     ND                                          B       19     --          PD     NS                                   11     A       19     --          S      ND                                          B       19     --          S      ND                                   12     A       19     --          SS     NDS                                  13     B       23     --          (3)    NDS                                  ______________________________________                                         (1)Solution viscosity in MEK (dl/g) as measured by the procedure describe     in ASTM D2857.                                                                 (2)Solubility before and after molding. Determined by visual inspection      after refluxing the sample in methyl ethyl ketone for 2 hours.                (3)Barely fusible. Cannot be satisfactorily thermoformed for measurement      of physical properties. The product was highly discolored.                    D = dispersible                                                               S = swells                                                                    PD = partially dispersible                                                    ND = not dispersible                                                          SS = swells slightly                                                          NDS = does not disperse or swell                                         

A sample of each polymer obtained was molded using standard compressionmolding techniques and the molded pieces were set aside for testing. Theresults of this testing are reported in Example 14. The solubility testwas performed before and after the molding operation to show the effectsof molding.

From Table 4 several significant facts emerge. The first is that as theAN content increases the tendency towards crosslinking increases asdemonstrated by the behavior when attempts are made to disperse thepolymer in boiling MEK. The solubility test is very severe indeed andindicates that the polymers characterized as ND are highly crosslinked.

The second point is that at the higher levels of AN the polymer can bemolded under conventional compression molding conditions but that aftermolding it has become so crosslinked that it will no longer even swell.It should be noted that Example 12 which was brabendered for 10 minutesat 240° C. could still be molded though it was too crosslinked to giveadequate fusion in the mold under the conditions used.

The third point is that even substantial amounts of chain transfer agentin Example 11 were unable to prevent crosslinking to the extent that thepolymer could no longer be dispersed though it was still moldable.

Finally, it can readily be seen that above about 20% of acrylonitrilethe crosslinking tendency becomes so strong that it becomes impossibleto perform the necessary drying and compounding operation for theproduction of a polymer that is still capable of being formed by heatand pressure because the polymer has become completely crosslinked.

EXAMPLE 14

In this Example the impact strengths and DTUL of the polymers ofExamples 2 to 11 are compared with the impact strength of a polymercontaining about the same level of rubber but with no acrylonitrile. Theresults are set out in Table 5.

                  TABLE 5                                                         ______________________________________                                        PROPERTIES OF POLYMERS                                                                Compo-                                                                        sition    %       Polymer       DTUL                                  Example S/MA/AN   Rubber  Separation                                                                            Izod(1)                                                                             (°C.)(2)                       ______________________________________                                        Compar- 76/24/0   13.7    A       114   121                                   ative                     B       114   121                                   2       68/26/6   14.3    A       112   136                                                             B        82   136                                   3       66/28/6   15.8    A       160   140                                                             B       170   141                                   4       68/26/6   15.4    A       161   138                                                             B       147   139                                   5       65/24/11  16.0    A       195   136                                                             B       239   131                                   6       69/20/11  14.0    A       157   124                                                             B       201   122                                   7       66/23/11  13.4    A       138     133.5                                                         B       150     130.5                               8       63/23/14  14.3    A       (Not moldable)                                                        B       151   --                                    9       66/20/14  12.2    A       185   126                                                             B       131   125                                   10      56/25/19  15.6    A       170     135.5                                                         B       181   135                                   11      56/25/19  13.5    A       174   136                                                             B       175   136                                   ______________________________________                                         (1)Notched Izod (J/m notch) as measured by the procedure described in AST     D256.                                                                         (2)Distortion Temperature under Load as measured by the procedure             described in ASTM D648.                                                  

From the data in Table 5 it can readily be seen that the Izod impactstrength of the polymer rises rapidly with the acrylonitrile content.This in itself is remarkable since crosslinking has not been shown toimprove toughness in any other known polymer system containing a rigidmatrix.

From the above data it can be seen that the present invention defines arange of compositions within the broad disclosure of the prior art thathave a unique capability for being thermoformed even after having beenexposed to a substantial heat history during production. The narrowrange of products thus defined also have unexpected and advantageousproperties that are not predictable on the basis of the prior art andare not shared by the homologues containing larger amounts ofunsaturated nitrile monomer.

The above Examples are for the purpose of illustration only and are notto be considered as implying any limitation on the essential scope ofthe invention. Minor variations and modifications of the compositionsand processes described herein are considered to be within the purviewof the invention.

What is claimed is:
 1. A process for the production of a polymericmolding composition which comprises polymerizing a monomer mixturecomprising from 50 to 83% by weight of a vinylaromatic monomer, from 15to 30% by weight of an unsaturated dicarboxylic acid anhydride and from2 to 20% by weight of an unsaturated nitrile in the presence of from 2to 30% by weight based on the composition weight of a rubber having aglass transition temperature below 0° C. to form a rubber substrategrafted with a superstrate terpolymer and dispersed in a matrixterpolymer; said process comprising forming a solution comprising thevinylaromatic monomer, the unsaturated nitrile monomer, a minor amountof the anhydride monomer and the rubber; catalytically initiatingpolymerization of the mixture at a temperature below 120° C., andmaintaining a low anhydride monomer concentration by gradual continuousaddition of the balance of the anhydride monomer during thepolymerization reaction so as to produce a rubber-modified terpolymer ofuniform composition that remains thermoformable after having beenexposed to temperatures of 190° C. for 30 minutes and thereafterBrabender compounded at 240° C. for 2 minutes.
 2. A process according toclaim 1 in which the reaction temperature is maintained below 100° C.and which employs an initiator selected from the group consisting oftert-butyl peracetate, tert-butyl peroctate, tert-butyl perbenzoate andmixtures thereof.
 3. A process according to claim 1 in which up to 1% byweight of a chain transfer agent is added to the polymerization mixture.4. A process according to claim 3 in which the chain transfer agent isselected from the group consisting of tertiary dodecyl mercaptan,terpinolene, myrcene, d-limonene, norbornene and indene.
 5. A processfor the production of a polymeric molding composition which comprisespolymerizing a monomer mixture comprising from 56 to 78% by weight ofstyrene, from 20 to 30% by weight of maleic anhydride and from 2 to 14%by weight of acrylonitrile in the presence of from 2 to 30% by weightbased on the composition weight of a diene rubber having a glasstransition temperature below -30° C. and a chain transfer agent to forma rubber substrate grafted with a superstrate terpolymer and dispersedin a matrix terpolymer; said process comprising forming a solutioncomprising the styrene, the acrylonitrile, a minor amount of the maleicanhydride and the diene rubber; catalytically initiating polymerizationof the mixture at a temperature below 120° C., maintaining a lowanhydride monomer concentration by gradual addition of the balance ofthe maleic anhydride during the polymerization reaction so as to producea rubber-modified terpolymer of uniform composition that remainsthermoformable after having been exposed to temperatures of 220° C. foran hour and thereafter Brabender compounded for 10 minutes at 240° C.